"Easy" mast stepping - alternatives for a tabernacle?

  • 11 Mar 2018 01:17
    Reply # 5955040 on 4611808

    The hinge

    It seems that fitting out the square tabernacle to take the round mast boils down to two approaches: Build up the mast to a square section (the packing being part of the mast) – call this method 1.

    Or, as David is suggesting, make a cast packing, in two parts, which acts as a clamp, one part attached to the tabernacle and neither part attached to the mast. – method 2.

    Both are practical and (on thinking about it) neither is difficult to do.

    I can see the beauty of method 2.

    If neither approach has any potential disadvantages, then, before deciding, it is also necessary to consider the type of hinging system to be employed, because there are implications. It is a given that there will be no welding or drilling of the mast at this point, or pins through the mast.

    Method 1 allows a simple and cheap hinging arrangement. For example (breaking the no-weld rule just once) a piece of heavy-walled tube can be welded across the aft upper end of the tabernacle, stiffening it, providing a roll-over surface to bear the flat aft of the built-up mast as it falls backwards over the tabernacle – and allows for a temporary pin to be pushed through, protruding each side, to make a temporary “sampson post” to which the mast can be lashed when raising or lowering. The lashing will rotate with the mast as it rolls back over into the lowered position, as the centre of rotation is the centre of the pin. I don’t think anything could be much simpler (apart from a suggestion David has made in the past which is to face the tabernacle aft and hinge the mast at the deck, with no fitting at all. I would certainly do it that way if I were not encumbered with a raised cabin preventing the mast from reaching the horizontal.)

    (Method 1 allows the attachment of a normal type of hinge to the mast too, as there would probably be enough meat in the packing to take screws - perhaps applicable to a smaller set up. But I won't be doing that.)

    David’s Method 2 does not, as far as I can see, lend itself so readily to such a simple hinge as the pin and lashing allowed by method 1.

    I am inclined to think that if there are reasons why method 2 is preferable, then it would pay to cast an extra pair of semicircle packers, to enable the making of a temporary clamp to be able to put around the mast just above the tabernacle, and a custom made fitting which can make this clamp the second part of a two part hinge, the first part being the removable pin through the welded-on tube, as described in method 1. It needs to be in place only when actually raising or lowering the mast.

    I think this would be very nice, but it might call for some more workshop engineering and a bit more cost and time. Just considering the hinge on its own, I think the rope lashing is a bit more KISS and I like it more.

    My guess is that one method is as good as the other, and hopefully not much difference in cost, so perhaps it just boils down to personal preference and what skills and materials are at hand.

    But I am not sure.

    Method 1 requires a nice fit, with the possibility of some clamping in the fore-and-aft direction by the use of a gasket placed under the bolt-on 4th side. (Thin polyurethane foam would be good here, I think.)

    Method 2 simply allows a good tight clamp all round (though with immediate release the moment the front is removed and the mast allowed to rotate backwards.)

    Are there any disadvantages to method 1?

    Last modified: 11 Mar 2018 03:52 | Anonymous member
  • 10 Mar 2018 23:17
    Reply # 5953797 on 4611808

    Thanks again for your technical advice, David, so generously given.

    I have not used castable polyurethane before, but will see if I can find out what it costs and where to get it here. I would like to learn how to use it, because it may well turn out to be the best way to join the two aluminium mast sections together, later. For the moment I feel a little under-confident in my ability to do it the way you have suggested and am still a little attracted to the less elegant approach, using the materials I am familiar with. Still thinking. A built up mast for a close fit - not hard to do - as opposed to David's suggestion of a tight clamp. Hmmm, might be better. I will report back when I have got my hands on some of this castable stuff and had a play with it.

    (David's advice to go for the larger version of the folded aluminium section has already proved advantageous. The 11mm clearance all round gives a useful working space in which to carry out the fitting of the mast to the tabernacle. I am continually grateful, and amazed at the value of this forum as a melting pot for ideas.)

    Last modified: 11 Mar 2018 01:19 | Anonymous member
  • 10 Mar 2018 20:31
    Reply # 5952197 on 5951234
    Annie Hill wrote:

    David, do you think there could be any issue with corrosion where you have water between the mast and the casting?

    I don't think so, because the whole idea is to make a clamped fit, thus excluding water. Corrosion only occurs when you have dissimilar metals with salt water lying between them. It doesn't occur either 1) when aluminium has full access to air, or 2) when everything that might act as an electrolyte is excluded. 
  • 10 Mar 2018 19:09
    Reply # 5951234 on 4611808

    David, do you think there could be any issue with corrosion where you have water between the mast and the casting?

  • 10 Mar 2018 09:28
    Reply # 5944849 on 4611808

    The channel is looking good, and seems to be a very cost effective and rapid way of making a tabernacle.

    I would go with option (3), as it is indeed quick and easy compared with the alternatives, and automatically achieves a good fit. 

    I would aim to make two semicircular components at top and bottom, with a small gap between them when assembled such that they will act as a clamp to hold the mast firmly in position with no annoying rattle.

    Here's a suggested way of proceeding. Lay the channel horizontal. Make eight thin plywood dams that fit into the channel, 85mm high, with a semicircular cutout to fit the mast just above the centre of channel, so with its centre 88mm above the bottom. Fix two of the dams in place with modelling clay, 100mm apart, at the top of the channel, and two more just above the where the deck level is going to be. Apply mould release wax liberally to the mast, at the points where it will lay in these dams. Now do the same again, in different positions, but this time, apply mould release wax liberally to the channel as well. Lay the mast into these dams, and seal around the dams with modelling clay. Pour castable polyurethane into the moulds thus formed, up to the top of the dams. Remove the mast, and the two mouldings that are going to fit on the open side. Screw these two mouldings to two pieces of flat bar which are then to be bolted firmly to the flanges when the mast has been stepped, with maybe packing pieces if needed, to clamp the mast. The anti-turning device can be a large screw through the lower, deck level clamp into the mast tube. 

    I would use something similar to Smooth-On 770, 780 or 790. Since this has proved to be fully able to support a mast through deck partners and at the heel, rather than wooden edges, it will work well in this application as well.

    Last modified: 10 Mar 2018 09:37 | Anonymous member
  • 09 Mar 2018 20:25
    Reply # 5936607 on 4611808

    Ok, I have got the channel made. It is 175mm x 175mm ID, wall thickness 6mm and 2.4m long.

    It weighs 23 kg and the price was very reasonable I thought: under NZ$250.

    The heel of the tabernacle will need to be extended 400mm to reach the keel, and above deck it will allow 1m bury.

    Above deck, the channel will have a 4th side comprising a plate bolted to the flange.

    The 10m mast will sit on a 250mm wood block, at deck level, bonded into the tabernacle box with Simpsons.

    Here is what it looks like, enclosing the mast pole.

    The mast is 5mm wall thickness, 153mm OD. So there is 11mm clearance all round.

    So far so good?

    The next question is fitting the mast to the tabernacle. Options?

    (1) Fitting the tabernacle to the mast: A simple way might be to line the inside of the tabernacle with 10mm ply Simpsoned to the aluminium,  with wooden fillets epoxied into the 4 corners of the box, and with square mast heel plug  so as to prevent rotation.

    (2) Fitting the mast to the tabernacle: Instead of (1) above, build the mast up to a square section to fit the tabernacle, at the top and bottom (over a length of, say, 200mm at the top and the bottom ?)

    For some reason this appeals to me more, though it might be more trouble.

    (There will be no rotation. Square is easier to fit than round. There is a good flat roll surface for the mast as it rotates over the back of the tabernacle.)

    As the build-up is really only a packing piece, would it be OK to make the build-up of wood, fibreglass and epoxy, in this situation? I am familiar enough with epoxy to be able to do this. 

    * I thought perhaps I could make a box out of 6mm ply, filleted, coved and gooped onto the mast with Simsons - then the outside moderately heavily coated with glass and epoxy to make it tough and to fill out the last 4-5mm all round.

    The way it is set up at the moment, the  channel could be used as a jig, to get it all square and lined up accurately while making.

    (3) The third option I was wondering if the mast and channel could be set up as in the above photo, accurately, temporary dams installed and a castable material poured into the channel, so as to simply cast the two square packings. I have noted with interest past postings by David in which he has described a way of joining two aluminium tubes using liquid castable, but I do not know exactly what material is called for or whether it is tough enough to be suitable in this situation. Maybe this would be quick and easy – is there a suitable liquid castable – and suitable parting agent – to make this a practical solution? The volume required for two square packings calculates out to about 2.0 – 2.5 litres. If it is frightfully expensive, maybe suitable pieces of wood could fill some of the volume.

    (2) * above seems the best way for me with what I have, but...

    again, I would gratefully appreciate opinions on this.

    Last modified: 09 Mar 2018 21:14 | Anonymous member
  • 02 Mar 2018 08:09
    Reply # 5884802 on 4611808

    Thanks David, your advice and comments are greatly valued.

    That just about wraps it up.

    I'll report back when it is done.

  • 02 Mar 2018 07:57
    Reply # 5884784 on 4611808

    I think I'd go with the larger version, on two counts:

    1) just supposing you weren't totally happy with the strength or stiffness of the mast, there would be room for a sleeve.

    2) the wider flanges ought to mean that no extra stiffening need be added.

  • 02 Mar 2018 00:41
    Reply # 5884538 on 4611808

    (Three hours later.)

    These guys are great to deal with.

    Here is the proposed section, to come exactly out of a 600mm wide half-sheet.

    The CAD drawing is here

    It comes out with an ID of 175mm square, and 44.5mm flange.

    Now that we know how much to allow for folding, it looks slightly too big to me.

    The sheet could be trimmed by a 25mm offcut - the dimensions would then be 170x170 with 39.5mm flange.

     However, I guess a few mm isn't going to make that much difference and simplest would be to run with it as it is, with no offcut.

    What do you think guys? As it is, or trim it down? (The sheet has not been cut yet.)

    Last modified: 02 Mar 2018 01:03 | Anonymous member
  • 01 Mar 2018 20:37
    Reply # 5884261 on 4611808

    Thanks Annie, yes, great to see your tabernacle in place, another step forward.

    I am starting to feel rather pleased with the aluminium folded top-hat channel idea, which, thanks to the valuable input on this forum has evolved into what I think will be a simple, cheap and workable alternative. I'm ordering the material today.

    But nothing beats the warmth of wood, and I am envious. 

    I went to the same yard where you got your wood, but it had been well picked over and only a couple unsuitable lumps were left. The last demolition firm I approached, looking for some oregon, told me "you should have asked last week, we just bulldozed a heap of it." They told me that when demolishing buildings they used to salvage on average 70% of all the wood. Today they salvage zero, because, they said, health and safety regulations and its associated bureaucracy have made it uneconomic, and it pays now to just use heavy equipment and send everything to the dump. I hope that's just an excuse for laziness and not the general trend. This is not the New Zealand I remember. 

    The aluminium tabernacle, with no welds, is going to work out slightly cheaper and quicker than yours - but not half as nice. I can't wait to see your boat on the water!

    Last modified: 01 Mar 2018 20:42 | Anonymous member
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software